SPECIFYING HOT DIP GALVANIZING - Perry Metal …

1 Hot Dip GALVANIZING by Perry Metal ProtectionSPECIFYING HOT DIP GALVANIZINGIt s simple to specify and more cost effectivethan alternative coating | INDUSTRIAL | INFRASTRUCTURE | RESIDENTIALWe ve got you coveredCOATING SELECTION PROCESSWhen selecting a corrosion protection coating the overall environment at the location of the structure requires careful consideration. A structure situated in an aggressive environment will require a much higher standard of corrosion protection than one in a benign environment. The environment can affect both the steel and the coating system. Of prime importance is the effect the environment has on the corrosion of following steps should be followed to select the most appropriate corrosion protection coating :Specified Intended Life Refer NZBC Clause B2 DurabilityThe NZ Building Code requires the detailing and SPECIFYING for durability of steel structures and members with a specified intended life of not less than 50 years for building structures.

2 A shorter life to first maintenance may be selected in conjunction with a maintenance programme which together will meet the durability provisions of NZBC Clause B2. Where components of the structure are not accessible for maintenance after assembly the coating must achieve the specified intended the Atmospheric Corrosion Category Refer :2009 and :2014 General atmospheric environments (macroclimate) are classified into six atmospheric corrosivity categories (C1 to C5 and CX) based on the corrosion rates of mild steel given in ISO 9223. In addition to climatic effects, the site specific environmental effects (microclimate) need to be considered. Factors requiring consideration include whether the steel surface is shaded, in a wet location, and whether the steel is in contact with timber or concrete.

3 The most significant microclimate effect is if the steel surface is sheltered from rain washing but exposed to the windblown marine salts as this greatly influences the corrosion the Life to First Maintenance Refer :2009 and :2014 The life to first maintenance available options takes into account the asset owners expectations, and the general principles of design to avoid corrosion. The ease of maintenance of the coated article if the life to first maintenance of the coated article is less than the required durability also requires AND DURABILITY OF HOT DIP GALVANIZINGHot dip GALVANIZING has proven to be more serviceable and predictable than all other steel protective coatings in the New Zealand atmosphere.

4 Its excellent performance is due to its inherent corrosion resistance, high tolerance to mechanical damage and inertness to the high UV levels prevailing over all of New wishing to specify hot dip GALVANIZING (HDG) need only use two Standards; one covering the design and durability of HDG steel AS/NZS :2014, and the other dealing with manufacturing process and tolerances AS/NZS 4680:2006. A detailed section on the design of duplex coatings (paint over HDG) is included, with two performance options for durability (aesthetic and corrosion).For engineers and fabricators the design details are extensive and pictorial advice on good design practice provides clear instruction. Appendices to the Standard also cover corrosion in different environments, including bimetallic corrosion and the interaction of HDG steel with soil, concrete, water, chemicals, and single table is provided in AS/NZS for designers to compare the expected durability of different galvanized products, allowing for a faster product selection process.

5 The durability of a HDG coating is calculated from the minimum average coating thickness in AS/NZS 4680, which also means non-standard HDG thicknesses can be easily assessed for estimated life to first maintenance. SPECIFYING HOT DIP GALVANIZINGR eference StandardArticle Thickness mmAverage coating ThicknessSelected Corrosivity Category (ISO 9223)Calculated life in yearsg/m2 mC1C2C3C4C5 CXHDG320 Less than + to + to +100+33-10016-338-162-8 HDG600 Over +100+40-10020-4010-203-10 HDG900 Over +900125100+100+60-10030-6015-305-15 The corrosion rate of a zinc coating is affected by the time for which it is exposed to wetness, air pollution and contamination of the surface. To a first approximation, the corrosion of all zinc surfaces is at the same rate in a particular environment.

6 A brief description of macro environment corrosivity categories are detailed C1 Generally dry indoors and some alpine C2 - External environments in this category are mostly areas beyond at least 50 km from the C3 - This category mainly covers coastal areas with low salinity. Along ocean front areas with breaking surf and significant salt spray, it extends from about 1 km inland to between 10 to 50 km inland, depending on the strength of prevailing winds and topography. This category also includes urban and industrial areas with low pollution C4 - This category occurs mainly on the coast and can extends from about several hundred metres inland to about one kilometre inland. Industrial regions may also be in this C5 - This category is common offshore and on the beachfront in regions of rough seas and surf beaches and can extend inland for several hundred metres.

7 This category may also include aggressive industrial areas, where the environment may be acidic with a pH of less than CX - These regions are found at some surf beach shoreline regions with very high salt deposition and are also be found in severe acidic industrial HOT DIP GALVANIZINGMANUFACTURING PROCESSES AND TOLERANCESAS/NZS 4680:2006 is the manufacturing standard for the hot dip GALVANIZING coatings on fabricated ferrous articles. The Standard includes requirements for coating mass and thickness, appearance, defect identification, and suitable repair 1 and Table 2 of AS/NZS4680 state the requirements for coating thickness and mass, which are based on the steel article Thickness mmMinimum Local coating Thickness mMinimumAverage coating Thickness mMinimum Average coating Mass g/m Less than to to +85125900 Table 2 Requirements for coating thickness and mass for articles that are centrifugedArticle Thickness mmMinimum Local coating Thickness mMinimumAverage coating Thickness mMinimum Average coating Mass g/m Less than than 1 Requirements for coating thickness and mass for articles that are not centrifugedSPECIFYING HOT DIP GALVANIZINGF reedom from defectsA galvanized coating should be continuous, adherent, as smooth and evenly distributed as possible.

8 And free from any defect that is detrimental to the stated end use of the coated article. The integrity of the coating can be determined by visual inspection and coating thickness measurements. A galvanized coating should be sufficiently adherent to withstand normal handling during transport and galvanized steel is a simple process. Zinc will not adhere to or react with unclean steel; therefore, a visual inspection of the product provides a good assessment of the quality of the coating . The coating thickness is usually tested using a magnetic thickness gauge. The testing and sampling requirements are contained in the appropriate specification for the product (AS/NZS 4680 and AS 1214). The :2014 Standard also includes a detailed section on the design of duplex coatings (paint over HDG), with two performance options for durability (aesthetic and corrosion).

9 A duplex system will increase the service life of the HDG article beyond that of the unpainted article. Further, the total life of a properly specified, applied and maintained duplex coating system is significantly greater than the sum of the lives of the HDG coating and the paint coating alone (by times, depending on the environment).AS/NZS includes seven decorative and industrial paint systems suitable for most corrosivity COATINGSP ainting for decorative, identifying colour or enhanced service lifeHot dip galvanized coatings are sometimes required to be painted for decorative reasons, to provide an identifying colour, or to enhance service life. In contrast with organic paints, which are degraded by solar radiation (UV), hot dip galvanized coatings are unaffected by sunlight, so over-painting is not usually required to extend service life in this circumstance.

10 The systems shown in the Standard, when applied and maintained correctly, will increase the service life of the hot dip galvanized article beyond that of the unpainted article. Preparation for paintingWhen painting hot dip galvanized coatings, as when painting any other surface, the cleanliness and condition of the surface are of critical importance and a high proportion of paint failures on hot dip galvanized coatings can be attributed to inappropriate or inadequate surface preparing hot dip galvanized coatings for painting, the basic requirements are largely the same as for other surfaces. Namely, anything that prevents the paint wetting out or adhering to the surface needs to be removed. Therefore, oils, dirt, dust, salts, corrosion products and other friable material and soluble salts have to be removed prior to sweep (brush) blast cleaning is a common method used for the preparation of a galvanized coating prior to the application of a paint coating .